The present invention relates to a temperature detecting apparatus for detecting the temperature of a target object. It also relates to a fixing apparatus for thermally fixing an unfixed toner image to recording medium, which uses a temperature detecting apparatus.
FIG. 5 is an enlarged schematic view of a fixing roller 1 and a temperature sensor 6. As shown in FIG. 5, the temperature detection element 12 of the temperature sensor 6 is embedded in the temperature detection surface of the a protective member 13, which remains in contact with the fixing roller 1. The protective member 12 is generally formed of a spongy substance. In order to prevent the surface of the fixing roller 1 and the temperature detection element 12 of the temperature sensor 6 from being damaged by the friction resulting from the contact between the temperature sensor 6 and fixing roller 1, the surface of protective member 13, in which the temperature detection element 12 is embedded, and the surface of the temperature detection element 12, which faces the fixing roller 1, are covered with a piece of protective tape 14, which is formed of a highly lubricous substance. As for the conventional material for forming the protective tape 14, polyimide or the like has been used. The thickness of the spongy protective member 13 is 6 mm, and the thickness of the protective tape 14 is 20 xcexcm. The temperature of the surface of the fixing roller 1 is detected by the temperature sensor 6, and the length of the time a heater 5 within the fixing roller 1 is kept turned on is controlled in response to the detected temperature so that the surface temperature of the fixing roller 1 remains at a target level.
The fixing roller 1 and an unshown pressure roller are kept pressed upon each other, forming a nip portion. As a piece of recording medium, onto which an unfixed toner image has just been transferred, is passed through the nip portion, the unfixed toner image is thermally fixed to the recording medium surface, turning into a permanent image.
In recent years, however, an image forming apparatus has drastically increased in speed. The higher the operational speed of an image forming apparatus, the greater the amount by which the surface temperature of the fixing roller drops as a substantial number of recording medium are continuously passed through the fixing apparatus, failing to conduct heat to the toner on the recording medium by a sufficient amount. Therefore, the so-called offset, that is, the phenomenon that toner adheres to the fixing roller, is likely to occur.
In the past, the offset toner adhered to the protective tape, which covered the temperature detection surface of the thermistor, and collected thereon. As the amount of the offset toner, which had collected on the protective tape covering the temperature detection surface of the thermistor, exceeded a certain amount, the collected offset toner dislodged, all at once, from the protective tape of the thermistor, and re-adhered to the toner image bearing surface of the recording medium which happened to be passing through the nip between the pressure roller and fixing roller, contaminating the image on the recording medium.
Further, since the offset toner adhered to the protective tape on the temperature detection surface, and collected thereon, there was a possibility that the surface temperature of the fixing roller would be incorrectly detected.
As a means for preventing the offset toner from adhering to the temperature detection surface of the thermistor, it is possible to place the temperature detecting member outside the recording medium path. This method, however, has its own problem. That is, in a case that a recording medium of a large size is passed immediately after a recording medium with a smaller size is passed, it is difficult to accurately estimate the temperature of the center portion of the fixing roller based on the temperature detected at the edge portion of the fixing roller. It is also possible to employ a noncontact type temperature detecting member. This method, however, is also problematic in that not only is a noncontact type temperature detecting member difficult to control but also high in cost.
The primary object of the present invention is to prevent toner from adhering to the temperature detection surface of the temperature sensor of a fixing member, so that it becomes possible to provide a fixing apparatus which does not suffer from the problems that an image is contaminated by the toner which collected on the temperature detection surface of the temperature sensor, and/or that the temperature of the fixing roller is incorrectly detected.
Another object of the present invention is to provide a temperature detecting apparatus, the temperature detection surface of which is covered with a glass fiber sheet coated with a sheet of resin film superior in releasing properties, across the surface which makes contact with an object, the temperature of which is to be detected.
Another object of the present invention is to provide a fixing apparatus comprising: a rotational member which makes contact with a sheet of recording medium bearing an unfixed image in order to fix the unfixed image to the recording medium: and a temperature sensor placed in contact with the rotational member to detect the temperature of the rotational member, wherein the surface of the temperature sensor, which faces the rotational member, is covered with a glass fiber sheet coated with a sheet of resin film superior in releasing properties, across the surface which faces the rotational member.
These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings.